Rivers of Hydrogen Gas May Fuel Spiral Galaxies

Three distinct features are visible in this composite image of NGC 6946. The bright heart of the galaxy in optical light (blue), the dense hydrogen in the spirals (orange), and the extended halo surrounding the galaxy (red). New research also shows a faint filament that could be hydrogen flowing from the intergalactic medium into the galaxy to fuel star formation.

Inpouring rivers of hydrogen gas could explain how spiral galaxies maintain the constant star formation that dominates their hearts, a new study reports.

Using the Green Bank Telescope (GBT) in West Virginia, scientists observed a tenuous filament of gas streaming into the galaxy NGC 6946, known as the "Fireworks Galaxy" because of the large number of supernovae observed within it. The find may provide insight into the source of fuel that powers the ongoing birth of young stars, researchers said.

"We knew that the fuel for star formation had to come from somewhere," study lead author D.J. Pisano, of West Virginia University, said in a statement. "So far, however, we've detected only about 10 percent of what would be necessary to explain what we observe in many galaxies." [Supernova Photos: Great Images of Star Explosions]

Located 22 million light-years from Earth on the border of the constellations Cepheus and Cygnus, NGC 6946 is a medium-sized spiral galaxy pointed face-on toward the Milky Way.

Previous studies revealed a halo of hydrogen gas around NGC 6946 common to spiral galaxies. Such halos are formed by hydrogen ejected from the galaxies by star formation and violent supernova explosions. These interactions heat the gas in the halo to extreme temperatures.

When Pisano turned the GBT toward the spiral galaxy for further examination, however, he discovered a ribbon of gastoo cool to have suffered the heating processes undergone by halo gas.

On average, the Milky Way churns out between 1 to 5 new stars per year. Rich in gas, NGC 6946 is far more active. For example, it has hosted at least 9 explosive supernovae in the past century.

The medium-sized spiral galaxy NGC 6946 has contained at least nine supernovae in the past century. Chandra observations (purple) have revealed three of the oldest supernovae ever detected in X-rays. Optical data from the Gemini Observatory is also included in this image (red, yellow, cyan).

Credit: X-ray: NASA/CXC/MSSL/R.Soria et al, Optical: AURA/Gemini OBs

"Starburst" galaxies are even more prolific. These productive galaxiesshould have burned through the gas they were born with over the course of their lifetimes, bringing star formation to a sudden halt. Instead, the process continues today, suggesting that something is continuing to supply them with sufficient gas to keep creating more stars.

"A leading theory is that rivers of hydrogen — known as cold flows — may be ferrying hydrogen through intergalactic space, clandestinely fueling star formation," Pisano said. "But this tenuous hydrogen has simply been too diffuse to detect, until now."

The immense, unblocked dish of the Green Bank Telescope, combined with its location in the National Radio Quiet Zone, where radio transmissions are limited, allow the large disk to detect the faint hydrogen signal that would be present in a cold flow.

Another possibility is that the hydrogen detected originated from a close encounter with another galaxy in the past. The gravitational interaction between the two could have stretched out a ribbon of neutral atomic hydrogen, researchers said. Such a ribbon would contain stars that astronomers should be able to easily observe, though none have yet been spotted. Further studies of the streamer hydrogen gas will help clarify its role.